Wen-Xin Tang

Running droplets of gallium from evaporation of gallium arsenide.

High-temperature annealing of gallium arsenide in vacuum causes excess evaporation of arsenic, with accumulation of gallium as liquid droplets on the surface. Using real-time in situ surface electron microscopy, we found that these droplets spontaneously run across the crystal surface. Running droplets have been seen in many systems, but they typically require special surface preparation or gradient forces. In contrast, we show that noncongruent evaporation automatically provides a driving force for running droplets. The motion is predicted and observed to slow and stop near a characteristic temperature, with the speed increasing both below and above this temperature. The same behavior is expected to occur during the evaporation of similar III-V semiconductors such as indium arsenide.

Asymmetric Spin-Wave Dispersion on Fe(110): Direct Evidence of the Dzyaloshinskii-Moriya Interaction

The influence of the Dzyaloshinskii-Moriya interaction on the spin-wave dispersion in an Fe double layer grown on W(110) is measured for the first time. It is demonstrated that the Dzyaloshinskii-Moriya interaction breaks the degeneracy of spin waves and leads to an asymmetric spin-wave dispersion relation. An extended Heisenberg spin Hamiltonian is employed to obtain the longitudinal component of the Dzyaloshinskii-Moriya vectors from the experimentally measured energy asymmetry.

Caustic imaging of gallium droplets using mirror electron microscopy

We discuss a new interpretation of mirror electron microscopy (MEM) images, whereby electric field distortions caused by surface topography and/or potential variations are sufficiently large to create caustics in the image contrast. Using a ray-based trajectory method, we consider how a family of rays overlaps to create caustics in the vicinity of the imaging plane of the magnetic objective lens. Such image caustics contain useful information on the surface topography and/or potential, and can be directly related to surface features. Specifically we show how a through-focus series of MEM images can be used to extract the contact angle of a Ga droplet on a GaAs (001) surface.

Laplacian image contrast in mirror electron microscopy

We discuss an intuitive approach to interpreting mirror electron microscopy (MEM) images, whereby image contrast is primarily caused by the Laplacian of small height or potential variations across a sample surface. This variation is blurred slightly to account for the interaction of the electrons with the electrical potential away from the surface. The method is derived from the established geometrical theory of MEM contrast, and whilst it loses quantitative accuracy outside its domain of validity, it retains a simplicity that enables rapid interpretation of MEM images. A strong parallel exists between this method and out of focus contrast in transmission electron microscopy (TEM), which allows a number of extensions to be made, such as including the effects of spherical and chromatic aberration.

Addendum. Laplacian image contrast in mirror electron microscopy

We extend the theory of Laplacian image contrast in mirror electron microscopy (MEM) to the case where the sample is illuminated by a parallel, collimated beam. This popular imaging geometry corresponds to a modern low energy electron microscope equipped with a magnetic objective lens. We show that within the constraints of the relevant approximations; the results for parallel illumination differ only negligibly from diverging MEM specimen illumination conditions.

Congruent evaporation temperature of GaAs(001) controlled by As flux

The congruent evaporation temperature Tc is a fundamental surface characteristic of GaAs and similar compounds. Above Tc the rate of As evaporation exceeds that of Ga during Langmuir (free) evaporation into a vacuum. However, during molecular beam epitaxy (MBE) there is generally an external As flux F incident on the surface. Here we show that this flux directly controls Tc. We introduce a sensitive approach to measure Tc based on Ga droplet stability, and determine the dependence of Tc on F. This dependence is explained by a simple model for evaporation in the presence of external flux. The capability of manipulating Tc via changing F offers a means of controlling congruent evaporation with relevance to MBE, surface preparation methods, and droplet epitaxy.

Asymmetric coalescence of reactively wetting droplets

Coalescence of droplets during reactive wetting is investigated for the liquid Ga/GaAs(001) system. In situmirror electron microscopy reveals that coalescence predominantly involves the motion of one reactive droplet relative to the other. This behaviour differs significantly from coalescence in non-reactive systems and is associated with contact line pinning at a ridge/etch pit edge which is identified using atomic force microscopy and selective etching. A simple geometrical model is presented to describe the pinning.

Ga droplet surface dynamics during Langmuir evaporation of GaAs

We describe the design and application of a low-energy electron microscope (LEEM) dedicated to the study of III-V materials. Recent studies of Langmuir (free) evaporation of GaAs(001) have been reviewed. Running Ga droplets are observed, and the motion is predicted and shown to slow and stop near a characteristic temperature. Striking bursts of Bdaughter[ droplet nucleation accompany the coalescence of large Bparent[ droplets. These observations imply that evaporation and surface morphology are intimately connected, suggesting a new approach for the self-assembly and positioning of nanostructures on patterned surfaces.

Planar regions of GaAs (001) prepared by Ga droplet motion

The authors describe a simple method for obtaining planar regions of GaAs (001) suitable for surface science studies. The technique, which requires no buffer layer growth, atomic hydrogen source, or the introduction of As flux, employs controllable Ga droplet motion to create planar trail regions during Langmuir evaporation. Low-energy electron microscopy/diffraction techniques are applied to monitor the droplet motion and characterize the morphology and the surface reconstruction. It is found that the planar regions exhibit atomic flatness at the level of a high-quality buffer layer.

Electron caustic lithography

A maskless method of electron beam lithography is described which uses the reflection of an electron beam from an electrostatic mirror to produce caustics in the demagnified image projected onto a resist–coated wafer. By varying the electron optics, e.g. via objective lens defocus, both the morphology and dimensions of the caustic features may be controlled, producing a range of bright and tightly focused projected features. The method is illustrated for line and fold caustics and is complementary to other methods of reflective electron beam lithography.